Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

The (o-p-o)-Terphenyl Embedded Calix[2]phyrin(2.2.1.1.1) and Its Cu(II) Complex

The introduction of a higher arene unit as a structural motif is unprecedented in calixphyrin chemistry. Herein, the syntheses, spectral and structural characterization of heretofore anonymous terphenylene unit (o-p-o) incorporated calixphyrin with fused sp2 meso carbons is reported. The explicitly tailored macrocyclic core is utilized to stabilize the Cu(II) metal ion. The molecular structures of the calixbenziphyrin and Cu(II) complex are unambiguously confirmed by single-crystal X-ray analysis. Moreover, theoretical supports uphold experimental conclusions.

Near-infrared absorbing nonaromatic core-modified meta-benzicalixhexaphyrin(1.1.1.1.1.1)s

Stable nonaromatic core-modified m-benzicalixhexaphyrins containing one m-phenylene ring, four pyrrole rings and one heterocyclic ring such as furan, thiophene, selenophene and telluorophene connected via four meso-sp2 carbons and two meso-sp3 carbons in a macrocyclic framework were synthesized. The m-benzitripyrrane dicarbinol was condensed with 16-heterocycle tripyrranes under mild acid-catalyzed and inert conditions followed by open-air oxidation with DDQ to obtain macrocycles in 2-5% yields. The presence of two -OH groups in the cis-orientation at two different meso-sp3 carbons, which are adjacent to the m-phenylene ring of the macrocycle, was confirmed through detailed 1D and 2D NMR studies. NMR studies indicated that the heterocyclic ring present across the m-phenylene ring prefers to be in the inverted conformation in these macrocycles. The macrocycles exhibited two intense absorption bands in the lower wavelength region of 320-580 nm and one broad absorption band that extended from the visible to NIR region, and the protonated derivatives of such macrocycles showed significant bathochromic shifts to the NIR region. Additionally, the electrochemical studies indicated that the macrocycles underwent easier oxidation due to their electron-rich nature. DFT studies revealed that the macrocycles adopted highly distorted conformation, which was consistent with experimental results.

Water-soluble anionic N-confused porphyrin for sensitive and selective detection of heavy metal pollutants in aqueous environment

Efficient detection and quantification of metal ions in real time and in a cost-effective manner is a critical step in combating the increasing danger of heavy metal contamination of our biosphere. The potential of water-soluble anionic derivative of N-confused tetraphenylporphyrin (WS-NCTPP) has been investigated for quantitative detection of heavy metal ions. The results show that the photophysical properties of WS-NCTPP differ significantly in the presence of four metal ions, namely Hg(II), Zn(II), Co(II) and Cu(II). The variation in the spectral behaviour is driven by the formation of 1:1 complexes with all the four cations with varied degree of complexation. The selectivity of the sensing is studied through interference studies, indicating maximum selectivity for Hg(II) cations. Computational studies of the structural features of the metal complexes with WS-NCTPP help in establishing the geometry and binding interactions between the metal ions and the porphyrin nucleus. The results demonstrate the promising potential of the NCTPP probe which should be utilized for detection of heavy metal ions, especially mercury, in the near future.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Metalloceneincorporated Hybrid Singly N-Methyl N-Confused Calixphyrins: Synthesis, Characterization, Protonation and Deprotonation Studies

Rational design and isolation of two hitherto unknown highly stable single conformer of ferrocene incorporated meso-aryl substituted singly N-methyl N-confused-calixphyrins have been achieved in quantitative yields. The solid-state crystal structure reveals the obvious trans-geometry for the meso-protons with the possibility for both the macrocycles to exist either racemic or enantiomer forms. However, thorough solution-state spectroscopic characterization strongly concludes the experimental isolation of a single isomer for both the macrocycles. The drastic modification of UV-vis spectral patterns upon imine pyrrole N protonation and amine pyrrole NH deprotonation of both the calixphyrins could pave way for these macrocycles to act as opto-electronic materials. The conformational preorganization and protonation and deprotonation induced conformational reorganization have been extensively studied by solution state spectroscopic techniques, solid state X-ray crystal structure and in depth DFT level theoretical calculations.

3-D Cryptand Like Normal and Doubly N-Confused Calixbenzophyrins: Synthesis and Structural Studies

The three-dimensional (3-D) cryptand-like normal and doubly-N-confused calixbenzophyrins embedded with phenyl rings in the macrocyclic skeleton are successfully synthesized from single starting materials. These structural isomers are characterized by spectral studies and unambiguously confirmed by crystal analyses.

Synthesis of Hydroxyisooxophlorins by Oxidative Degradation of meso-Hydroxyporphyrins

Hydroxyisooxophlorin is an oxidatively degraded porphyrin that contains one carbonyl carbon, one hydroxymethylene group, and two sp²-methine groups at its meso positions. Here we report that oxidation of a free-base meso-hydroxyporphyrin with [bis(trifluoroacetoxy)iodo]benzene in the presence of H₂O afforded two regioisomers of hydroxyisooxophlorins, 10-hydroxy-10-iso-5-oxophlorin and 15-hydroxy-15-iso-5-oxophlorin, as major and minor products, respectively. We also examined the reaction mechanism, acid-mediated isomerization, metal complexation behavior, and physical properties of these molecules.

meso-Aryl substituted stable unorthodox 5,10-porphodimethenes with α,β and β,β-N-methyl pyrrole connectivities: synthesis and spectroscopic, solid state and theoretical characterization

meso-Aryl substituted highly stable single conformers of doubly mutant porphodimethenes.

Functional Calixphyrins: Synthetic Strategies and Applications

Calixphyrins are hybrid macrocycles that contain both sp²- and sp³-hybridized carbon atoms and hence bear analogy to both porphyrins and calixpyrroles. Due to the presence of sp³-hybridized carbon atoms, π-conjugation is disrupted in calixphyrins, which leads to conformational flexibility. Hence, these molecules find a use in anion binding, host-guest chemistry and metal-coordination chemistry. During the last decade, studies on calixphyrins have been a topic of wide interest to researchers. Various functionalities have been introduced to the structure of calixphyrins, leading to the development of expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins with diverse applications. This review outlines the detailed historical origin of the synthesis of calixphyrins with emphasis on current research and development in this area. The modular syntheses of normal calixphyrins, expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins are also discussed, along with their applicative aspects.

pH-Regulated Reversible Transition Between Polyion Complexes (PIC) and Hydrogen-Bonding Complexes (HBC) with Tunable Aggregation-Induced Emission

The mimicking of biological supramolecular interactions and their mutual transitions to fabricate intelligent artificial systems has been of increasing interest. Herein, we report the fabrication of supramolecular micellar nanoparticles consisting of quaternized poly(ethylene oxide)-b-poly(2-dimethylaminoethyl methacrylate) (PEO-b-PQDMA) and tetrakis(4-carboxylmethoxyphenyl)ethene (TPE-4COOH), which was capable of reversible transition between polyion complexes (PIC) and hydrogen bonding complexes (HBC) with tunable aggregation-induced emission (AIE) mediated by solution pH. At pH 8, TPE-4COOH chromophores can be directly dissolved in aqueous milieu without evident fluorescence emission. However, upon mixing with PEO-b-PQDMA, polyion complexes were formed by taking advantage of electrostatic interaction between carboxylate anions and quaternary ammonium cations and the most compact PIC micelles were achieved at the isoelectric point (i.e., [QDMA(+)]/[COO(-)] = 1), as confirmed by dynamic light scattering (DLS) measurement. Simultaneously, fluorescence spectroscopy revealed an evident emission turn-on and the maximum fluorescence intensity was observed near the isoelectric point due to the restriction of intramolecular rotation of TPE moieties within the PIC cores. The kinetic study supported a micelle fusion/fission mechanism on the formation of PIC micelles at varying charge ratios, exhibiting a quick time constant (τ1) relating to the formation of quasi-equilibrium micelles and a slow time constant (τ2) corresponding to the formation of final equilibrium micelles. Upon deceasing the pH of PIC micelles from 8 to 2 at the [QDMA(+)]/[COO(-)] molar ratio of 1, TPE-4COOH chromophores became gradually protonated and hydrophobic. The size of micellar nanoparticles underwent a remarkable decrease, whereas the fluorescence intensity exhibited a further increase by approximately 7.35-fold, presumably because of the formation of HBC micelles comprising cationic PQDMA coronas and PEO/TPE-4COOH hydrogen-bonded cores, an inverted micellar structures compared to initial PIC micelles. Moreover, the pH-mediated schizophrenic micellar transition from PIC to HBC with tunable AIE characteristic was reversible.

Synthesis of 21,23-selenium- and tellurium-substituted 5-porphomethenes, 5,10-porphodimethenes, 5,15-porphodimethenes, and porphotrimethenes and their interactions with mercury

The 3+1 condensation of symmetrical 16-Selena/telluratripyrranes with symmetrical selenophene-2,5-diols/tellurophene-2,5-diols in the presence of BF3-etheratre or BF3-methanol followed by oxidation with DDQ gave 5,10-porphodimethenes, whereas the process with unsymmetrical selenophene-2,5-diols/tellurophene-2,5-diols gave 5-porphomethenes. In addition, the reaction of unsymmetrical 16-Selena/telluratripyrranes with symmetrical selenophene-2,5-diols/tellurophene-2,5-diols gave the corresponding porphotrimethenes, whereas the process with unsymmetrical selenophene-2,5-diols/tellurophene-2,5-diols gave the 5,15-porphodimethenes. The structures of different products were characterized by IR, (1)H and (13)C NMR, (1)H-(1)H COSY, CHN analysis, and mass spectrometry. The binding of mercury with the calix[4]phyrins mentioned above had been observed in the decreasing order of porphodimethenes > porphomethenes > porphotrimethenes by UV-vis and (1)H NMR spectroscopy.

Nanoparticles made of π-conjugated compounds targeted for chemical and biological applications

This feature article summarizes the recent applications of nanoparticles made of π-conjugated compounds in bio/chemo-sensing, disease therapy, and photoacoustic imaging.

Aggregation-induced emission: the whole is more brilliant than the parts

"United we stand, divided we fall."--Aesop. Aggregation-induced emission (AIE) refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents as molecules but become highly luminescent when they are clustered in poor solvents or solid state as aggregates. In this Review we summarize the recent progresses made in the area of AIE research. We conduct mechanistic analyses of the AIE processes, unify the restriction of intramolecular motions (RIM) as the main cause for the AIE effects, and derive RIM-based molecular engineering strategies for the design of new AIE luminogens (AIEgens). Typical examples of the newly developed AIEgens and their high-tech applications as optoelectronic materials, chemical sensors and biomedical probes are presented and discussed.

The syn and anti isomers of the porphyrinogen-like precursor of calix[4]phyrin: isolation, X-ray structure, anion binding and fluoride-ion-mediated proton-deuterium exchange studies

Porphyrinogen-like precursors of calix[4]phyrins are presumed to be unstable owing to their auto-oxidation. In contrast to this, the syn and the anti isomers of a calix[4]pyrrole molecule containing pyridine moieties at the meso positions were isolated and their structures were determined by single crystal X-ray diffraction studies. Both the isomers gave the same calix[4]phyrin molecule upon oxidation. The anion binding properties of both the isomers were studied in DMSO-d6 by the EQNMR method, which showed that they have a preference of binding with the F(-) ion over the other large sized halide and oxo anions. In addition, the F(-) ion mediated H/D exchange process was monitored by the (19)F NMR method. The solution state structures of the 1 : 1 F(-) ion complexes containing deuterium atoms formed by a random but sequential substitution of NH protons by deuterium atoms were identified from their multiplicity patterns observed in the proton coupled (19)F NMR spectrum, which are supported by the proton decoupled (19)F NMR spectrum showing one singlet for each type of F(-) ion complex in solution for both the syn and anti isomers, correlating with their solid state structures.